1. Field of the Invention
The invention concerns edge layer refinement of functional components. The present invention is useful in all functional components subjected to wear caused by sliding friction made of titanium or its alloys, which are stressed at operating temperatures below 500.degree. C., are subjected to high surface pressure, and must have as low a coefficient of friction as possible. The invention can be used particularly advantageously for the protection of human implants, in particular with oscillating movements, as well as aerospace sector components subjected to wear caused by sliding friction.
2. Discussion of Background
Titanium is an excellent construction material whose high specific strength, chemical resistance, and biocompatibility make titanium suitable for various special applications. However, titanium's low resistance to wear caused by sliding friction and its high coefficient of friction often prevent a broader range of use.
It is known to produce very wear-resistant edge layers on titanium by laser gas alloying (cf., e.g., H.W. Bergmann: "Thermochemical Treatment of Titanium and Titanium Alloys by Laser Melting and Gas Alloying", Zeitschrift fur Werkstofftechnik 16 (1985), p. 392-405).
Moreover, it is known to use laser gas alloying for the protection of joint endoprostheses (DE 3 917 211). For this, the component is melted by the laser beam to a depth of 0.1 to 0.7 mm, and nitrogen is simultaneously blown into the melt. Because of the high affinity of titanium for reactive gases, titanium nitride, which precipitates in the form of needles from the melt, forms immediately. After solidification, the edge layer consists of the metallic matrix of titanium with an altered .alpha./.beta. proportion compared to the initial state, as well as very densely embedded titanium nitride needles. The hardness of the edge layer is usually up to 1000 HV.
However, the shortcoming of such layers include a high coefficient of friction and, moreover, strong abrasive wear with most mating bodies which may be used. In this regard, the very hard titanium nitride needles protrude out of the surface after the initial wear. Thus, the local stress of the tribosystem is increased until the mating body is grooved and simultaneously microscopic interlocking of the titanium nitride needles with the mating body results, which increases the coefficient of friction.
Another shortcoming of these layers appears under loading in an oxygen-containing atmosphere and especially under relatively high temperatures and is expressed in that, particularly under deficient lubrication conditions, a catastrophic failure of the frictional pair may occur. The cause for this failure involves the metallic matrix between the TiN needles havings a high affinity for oxygen.
In order to circumvent the negative effects of the TiN needles, in particular for the human implant sector, a process for gas nitriding (U.S. Pat. No. 5,326,362) has become known in which molecular nitrogen is diffused into the region near the surface at a process temperature of 400.degree. C. to 704.4.degree. C. and forms a wear-resistant layer by solution hardening. For this, the component is placed in a vacuum furnace, evacuated to a pressure of 1.multidot.10.sup.-6 Torr, then filled with 1 atm nitrogen, heated to 537.7.degree. C.; the nitrogen pressure is increased to 2 atm, and the component is nitrided at 593.3.degree. C. for several hours. After completion of the treatment, the edge layer comprises a 0.2 .mu.m-thick compound layer of titanium nitrides, titanium carbon nitrides, titanium oxides, and titanium carbo-oxides and a diffusion layer a few .mu.m thick. The titanium nitrides found in the compound layer are significantly more finely dispersed than with laser gas alloying. Since the compound layer forms a closed layer on the surface, the loading capacity of the layer under an oxygen-containing atmosphere and elevated temperatures is increased.
The shortcomings of this process include that the friction coefficient is not adequately reduced and that the wear resistance is inadequate at high contact pressures. The shortcomings result from the fact that, on the one hand, the compound layer still comprises a very hard and not completely flat titanium nitride needles, which interlock with the mating body and, on the other hand, the underlying diffusion layer is too thin to be able to resist high local stress for an adequately long time. The primary reason for the latter is that with Hertz-calculated stresses with the contact surfaces appearing in actual practice, the maximum stress lies under the layer. Consequently, deformations may occur in the soft base material, which result in a lifting of the brittle compound layer.